1. Field of the Invention
This invention relates to a process for producing a toner used in image-forming processes such as electrophotography, electrostatic recording, electrostatic printing and toner jet recording.
2. Related Background Art
In image-forming processes such as electrophotography, toners for developing electrostatic latent images are used. Processes for producing such toners are roughly grouped into a pulverization process and a polymerization process. The pulverization process is available as a simple and popular production process. As a common production method therefor, a binder resin for the fixing to transfer mediums and a colorant for bringing out the color as a toner are used, to which a charge control agent for imparting electric charges to toner particles, a magnetic material for imparting transport properties to the toner itself, and additives such as a releasing agent and a fluidity-providing agent are optionally added, and these are mixed and melt-kneaded, then the kneaded product formed is cooled to solidify and thereafter pulverized by a fine-grinding means, optionally followed by classification to provide the desired particle size distribution and further addition of a fluidity-providing agent to produce a toner used for image formation. Also, in the case of toners used in two-component development, the above toner is blended with a magnetic carrier of various types, and the developer obtained is used for image formation.
As means for pulverization, various grinding machines are used. A jet mill, in particular, an impact jet mill, making use of an air jet as shown in FIG. 5 is often used. In the impact jet mill, a powder material is transported with a high-pressure gas such as an air jet and is jetted from an outlet of an acceleration tube and caused to collide against the collision surface of a colliding member so provided as to face the opening plane of the outlet of the acceleration tube, to pulverize the powder material by the impact force thus produced.
In the impact jet mill shown in FIG. 5, a colliding member 164 is so provided as to face an outlet 163 of an acceleration tube 162 to which a high-pressure gas feed nozzle 161 is connected. By the aid of high-pressure gas fed to the acceleration tube 162, a pulverizing material is sucked into the acceleration tube 162 from a pulverizing material feed opening 165 made to communicate with the acceleration tube 162 at its middle portion, thus the pulverizing material is jetted together with the high-pressure gas and caused to collide against a collision surface 166 of the colliding member 164. The material is pulverized by the impact thereby produced, and the resultant pulverized product is discharged from a pulverized-product discharge outlet 167.
In the above impact jet mill, however, a large quantity of air (high-pressure gas) is required in order to produce toners with small particle diameter. Hence, the apparatus requires a very large power consumption, and has a problem in respect of energy cost. In particular, in recent years, it is sought to achieve the energy saving of apparatus so as to cope with environmental problems.
As a countermeasure therefor, as a grinding machine which is more efficient than the jet mill in respect of energy, a rotary mechanical grinding machine as shown in FIG. 1 is used, which is disclosed in, e.g., Japanese Patent Application Laid-Open Nos. 59-24855 and 59-105853 and Japanese Patent Publication No. 3-15489. In this rotary mechanical grinding machine, the powder material is pulverized by introducing it to a circular space formed between a rotor which rotates at a high speed and a stator disposed around the rotor. According to this rotary mechanical grinding machine, the powder material can be pulverized at an energy much more saved than the jet mill, and moreover may be less over-pulverized, so that any fine powder or ultrafine powder may less occur, making it possible to improve yield.
Taking note of the shapes of toner particles obtained by pulverization using these grinding machines, it is known that the toner particles obtained by the jet mill have an amorphous (lacking definite form) and angular shape and that the toner particles obtained by the rotary mechanical grinding machine have a roundish shape. This is considered due to the difference in the manner of pulverization. More specifically, this is because, in the pulverization utilizing the air jet, the greater part of the powder material is pulverized by collision of particles one another or by their collision against the colliding member, whereas, in the pulverization using the rotary mechanical grinding machine, the greater part of the powder material is pulverized by collision of particles against the wall surfaces of the rotor and liner which rotate at a high speed. Also, in the rotary mechanical pulverization, the pulverization not a little causes heat generation to bring about an additional effect of being made spherical due to heat, so that the toner particles obtained by this pulverization may have a roundish shape, as so considered.
Hence, the toner particles obtained by pulverization using the rotary mechanical grinding machine have a smaller specific surface area than the toner particles obtained by pulverization using the jet mill, and hence have a good fluidity, also having so small voids between particles that they have superior packing properties and can afford addition of external additives in a small quantity. The former has such advantages. There may also an additional advantage on product quality such that the former has superior chargeability and transfer performance. Namely, the rotary mechanical grinding machine enables production of good-quality toners at a saved energy and in a high yield.
However, mechanical grinding machines including such a high-speed rotary fine grinding machine have caused problems that their continuous use or any inorganic matter contained in toner particles may make grinding surfaces of the grinding machine wear to cause a lowering of pulverization capacity, a change in quality of the pulverized product or a lowering of reliability because of inclusion of wear residues. What the grinding surfaces refer are the outer periphery surface of the rotor and the inner periphery surface of the stator. Thus, there has been no choice but changing for new ones where the surfaces have worn.
Especially in recent years, it has become general to use magnetic toners as one-component developers on account of their convenience. When such magnetic toners are produced by pulverization using the above rotary mechanical grinding machine, the surfaces of the rotor and stator may wear markedly faster than when toners containing no magnetic materials are produced. As the grinding surfaces wear, the shape of toner particles tends to become non-uniform and it becomes difficult to produce toner particles stably, so that the rotor and the stator may have so short lifetime that they must frequently be changed for new ones, resulting in an increase of product cost.
Accordingly, in order to make the rotor and stator have surfaces improved in wear resistance, it is attempted to treat their base members by quenching, carburizing or nitriding. There, however, are problems that the layer thus hardened has a small thickness and that the treatment is made at so high a temperature as to cause strain. Also, the surface hardness is not so high, and has been insufficient for the wear resistance required especially when magnetic-toner materials are pulverized.
It has also been attempted to coat base member surfaces of the rotor and stator with a ceramic material by spray coating. However, the coating layers formed tend to peel off and has been insufficient for maintaining the wear resistance.
Japanese Patent Application Laid-Open Nos. 7-155628 and 11-221480 disclose a method in which the base member surfaces of the rotor and stator are treated by lining with a titanium material to improve their wear resistance. This method certainly has an advantage that a high surface hardness can be achieved, but has had a problem that gaps tend to be made at the time of lining treatment to tend to cause peeling or cracking. It also has had a problem that materials for surface treatment are expensive.
An object of the present invention is to provide a toner production process which has solved the above problems and can produce a toner in a high throughput and a high yield.
Another object of the present invention is to provide a process for producing a toner which has good developing performance and transfer performance even in a high-temperature high-humidity environment and a low-temperature low-humidity environment and may less cause fog and spots around line images.
In order to solve the above problems the prior art has had, the present inventors made extensive studies. As the result, they have discovered that the surface treatment of the grinding surfaces of the rotor and stator in the rotary mechanical grinding machine correlates with wear resistance, pulverization capacity and toner""s developing performance, and have accomplished the present invention.
That is, the present invention is a toner production process having at least the steps of:
melt-kneading a mixture containing at least a binder resin and a colorant to obtain a kneaded product;
cooling the kneaded product to obtain a cooled product;
crushing the cooled product to obtain a crushed product;
pulverizing a powder material comprising the crushed product, by means of a grinding means to obtain a pulverized product; and
classifying the pulverized product by a classifying means;
wherein;
the grinding means comprises a mechanical grinding machine having at least a rotor which is a rotator attached to the center rotating shaft and a stator which is provided around the rotor, keeping a constant gap between the stator and the rotor surface; the grinding machine being so constructed that a circular space formed by keeping the gap stands airtight; and
the surface of at least one of the rotor and the stator is coated by the plating of a chromium alloy containing at least chromium carbide.